1. Field of the Inventions
The present inventions relate to enhanced heat transfer tubes which can be used as, for example, heat exchange elements of evaporators and condensers for refrigeration and air conditioning systems, as well as manufacture methods for the heat transfer tube, which falls into the technical field of heat exchange element and manufacture technique.
2. Description of the Related Art
Liquid boiling or condensing on outer surfaces of tube bundles is involved in the fields of refrigeration, air conditioning, process engineering, petrochemical industry, and energy and power engineering. For the evaporators and condensers used in refrigeration and air conditioning systems, the thermal resistance of phase change heat exchange during refrigerant boiling or condensing on the outer surfaces of the tubes is equivalent to or even larger than that of forced convection heat exchange inside the tube. Thus, significant improvement to heat transfer performance of evaporator and condenser can be achieved by enhancing phase change heat exchange on the outer surfaces of such tubes.
The study of nucleate boiling shows that boiling of a liquid requires the existence of nucleation sites. For a heating surface with given superheating temperature, only when the radius of the nucleation site is larger than minimum radius required by the vapor bubble growth of the liquid, a vapor bubble can grow, and hence nucleating boiling can be carried out. Cavities formed by grooves and/or cracks on the heating surface are more likely to become nucleation sites.
During boiling processes, after vapor bubbles grow and leave the cavities, some vapor is retained by the cavities and is not completely expelled by liquid flowing into the cavities. These retained amounts of vapor becomes new nucleation sites, growing new vapor bubbles to continue the boiling process. Thus, one way to enhance nucleation boiling heat exchange is to form more nucleation sites on the heating surface.
Since the 1970s, many developments for enhancing boiling heat transfer surfaces have been directed to the formation of numerous structures on heating surface, which can be found in many publications. For example, a heat exchange tube for evaporator disclosed in Chinese patent ZL95246323.7 (publication number CN2257376Y) and ZL03207498.0 (publication number CN2662187Y) has helical fins with top thereof pressed into a T-shape on its outer surface, so as to form channel structures. A heat exchange tube disclosed in Chinese patent ZL95118177.7 (publication number CN1090750C) and ZL02263461.4 (publication number CN2557913Y) has helical fins with inclined teeth uniformly distributed along the circumferential direction, in which cavity structures are formed by pressing the fin to make tooth top of the fin extend toward two sides. A heat exchange tube disclosed in Chinese patent CN1366170A (application number 02101870.7) has fins formed by machining on its outer surface, and secondary channels formed at bottom of the primary channels among the fins. A heat exchange tube disclosed in Chinese patent CN1100517A (application number 94116309.1) has fins on its outer surface pressed to be inclined toward one side, and cavity structures are formed by impressing notches on the shoulder part of the fins. Another heat exchange tube for evaporator is disclosed in ZL02264793.7 (publication number CN2572324Y) which has helical fins with sawtooth structures formed by machining on its outer surface, and cavity structures are formed by impressing inclined notches on the sawtooth top. A heat exchange tube disclosed in Chinese patent CN1731066A (application number 200510041468.6) has fins and transverse spikes formed by machining on its outer surface to form composite cavity structures.
In the aforementioned references, the outer wall surfaces, which are also referred to as “outer fin structures”, of the heat transfer tubes have a common structural feature that the heat transfer tubes are provided with channels or cavities with slightly small openings to constitute nucleation sites or carriers for evaporating, so as to enhance effect of the boiling heat transfer.
In recent years, structures on fin surfaces has been further explored to achieve more nucleation sites, so as to provide heat transfer tubes with further improved heat exchange coefficients of phase change heat exchange. This type of heat transfer tube can greatly increase heat exchange area without reducing mechanical strength of the enhanced heat transfer tube even in absence of phase change heat transfer, can enhance disturbance of fluid flowing by the heat exchange surface, and can reduce velocity boundary layers and temperature boundary layers, thus improving heat exchange performance and reducing weight of the heat transfer tube to certain extent.
However, in the disclosed references, there is no suggestion for outer fin structure on the outer tube of the heat transfer tubes for evaporators and condensers which can improve the phase change heat exchange coefficient significantly. Additionally, there is no suggestion of a process through which a phase change heat exchange coefficient can be significantly improved by machining outer fin surface of the enhanced heat transfer tube via appropriate mechanical machining.